Well, I am well versed with Crawford's regiment, but since the info you had states 15th Ark, I am still thinking he may have been in Buster's BN. What little info exists does show it was there. In the ATM especially with cavalry, men joined units that I have found no info on whatsoever except snipits from CSR's etc... Webb's cavalry was one of these, but not sure how long it existed and not much info on it.. much like Dodd's Union Spy company, and Harris's Guerilla Company, all operating in Saline Co. So, the only Murdock in a cavalry unit was the man who left the 26th and entered WEbb's.